Improved feature ergonomics

cardano-api/gen/Test/Gen/Cardano/Api/Typed.hs

@@ -7,8 +7,11 @@
 {-# LANGUAGE TupleSections #-}
 {-# LANGUAGE TypeApplications #-}
 
+{-# OPTIONS_GHC -Wno-deprecations #-}
+
 module Test.Gen.Cardano.Api.Typed
-  ( genFeatureValueInEra
+  ( genFeaturedInEra
+  , genMaybeFeaturedInEra
 
   , genAddressByron
   , genAddressInEra
@@ -731,16 +734,25 @@ genTxBody era = do
     Left err -> fail (displayError err)
     Right txBody -> pure txBody
 
--- | Generate a 'FeatureValue' for the given 'CardanoEra' with the provided generator.
-genFeatureValueInEra :: ()
+-- | Generate a 'Featured' for the given 'CardanoEra' with the provided generator.
+genFeaturedInEra :: ()
+  => Alternative f
+  => feature era
+  -> f a
+  -> f (Featured feature era a)
+genFeaturedInEra witness gen =
+  Featured witness <$> gen
+
+-- | Generate a 'Featured' for the given 'CardanoEra' with the provided generator.
+genMaybeFeaturedInEra :: ()
   => FeatureInEra feature
   => Alternative f
   => f a
   -> CardanoEra era
-  -> f (FeatureValue feature era a)
-genFeatureValueInEra gen =
-  featureInEra (pure NoFeatureValue) $ \witness ->
-    pure NoFeatureValue <|> fmap (FeatureValue witness) gen
+  -> f (Maybe (Featured feature era a))
+genMaybeFeaturedInEra gen =
+  featureInEra (pure Nothing) $ \witness ->
+    pure Nothing <|> fmap Just (genFeaturedInEra witness gen)
 
 genTxScriptValidity :: CardanoEra era -> Gen (TxScriptValidity era)
 genTxScriptValidity era = case txScriptValiditySupportedInCardanoEra era of

cardano-api/internal/Cardano/Api/Feature.hs

@@ -1,64 +1,46 @@
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE KindSignatures #-}
-{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StandaloneDeriving #-}
 
+{-# OPTIONS_GHC -Wno-deprecations #-}
+
 module Cardano.Api.Feature
-  ( FeatureValue (..)
-  , valueOrDefault
-  , asFeatureValue
-  , asFeatureValueInShelleyBasedEra
-  , isFeatureValue
+  ( Featured (..)
+  , asFeaturedInEra
+  , asFeaturedInShelleyBasedEra
   ) where
 
 import           Cardano.Api.Eras
 
--- | A value of type @'FeatureValue' feature era a@ is either:
-data FeatureValue feature era a where
-  -- | A value is available for this feature in this era
-  FeatureValue
+-- | A value only if the feature is supported in this era
+data Featured feature era a where
+  Featured
     :: feature era
     -- ^ The witness that the feature is supported in this era
     -> a
     -- ^ The value to use
-    -> FeatureValue feature era a
-
-  -- | No value is available for this feature in this era
-  NoFeatureValue
-    :: FeatureValue feature era a
-
-deriving instance (Eq a, Eq (feature era)) => Eq (FeatureValue feature era a)
-deriving instance (Show a, Show (feature era)) => Show (FeatureValue feature era a)
+    -> Featured feature era a
 
--- | Determine if a value is defined.
---
--- If the value is not defined, it could be because the feature is not supported or
--- because the feature is supported but the value is not available.
-isFeatureValue :: FeatureValue feature era a -> Bool
-isFeatureValue = \case
-  NoFeatureValue -> False
-  FeatureValue _ _ -> True
+deriving instance (Eq a, Eq (feature era)) => Eq (Featured feature era a)
+deriving instance (Show a, Show (feature era)) => Show (Featured feature era a)
 
--- | Get the value if it is defined, otherwise return the default value.
-valueOrDefault :: a -> FeatureValue feature era a -> a
-valueOrDefault defaultValue = \case
-  NoFeatureValue -> defaultValue
-  FeatureValue _ a -> a
+instance Functor (Featured feature era) where
+  fmap f (Featured feature a) = Featured feature (f a)
 
 -- | Attempt to construct a 'FeatureValue' from a value and era.
 -- If the feature is not supported in the era, then 'NoFeatureValue' is returned.
-asFeatureValue :: ()
+asFeaturedInEra :: ()
   => FeatureInEra feature
   => a
   -> CardanoEra era
-  -> FeatureValue feature era a
-asFeatureValue value = featureInEra NoFeatureValue (`FeatureValue` value)
+  -> Maybe (Featured feature era a)
+asFeaturedInEra value = featureInEra Nothing (Just . flip Featured value)
 
 -- | Attempt to construct a 'FeatureValue' from a value and a shelley-based-era.
-asFeatureValueInShelleyBasedEra :: ()
+asFeaturedInShelleyBasedEra :: ()
   => FeatureInEra feature
   => a
   -> ShelleyBasedEra era
-  -> FeatureValue feature era a
-asFeatureValueInShelleyBasedEra value = asFeatureValue value . shelleyBasedToCardanoEra
+  -> Maybe (Featured feature era a)
+asFeaturedInShelleyBasedEra value = asFeaturedInEra value . shelleyBasedToCardanoEra

cardano-api/src/Cardano/Api.hs

@@ -26,9 +26,11 @@ module Cardano.Api (
     cardanoEraConstraints,
     InAnyCardanoEra(..),
 
-    -- * Features
+    -- * Feature support
     FeatureInEra(..),
-    FeatureValue(..),
+    Featured(..),
+    asFeaturedInEra,
+    asFeaturedInShelleyBasedEra,
 
     -- ** Shelley-based eras
     ShelleyBasedEra(..),